A performance of an antenna can be defined in terms of its gain, bandwidth, antenna pattern, pattern control and reactance. A gain of an antenna can be defined as the ratio between a radiation intensity of the antenna in a certain direction and the radiation intensity that would be obtained if the power accepted by the antenna were radiated isotropically. A bandwidth of an antenna can be defined as a range of frequencies, on either side of a center frequency (usually the resonance frequency for a dipole) where the matching antenna characteristics (the input impedance) are within an acceptable value. The antenna fractional bandwidth is the ratio of the bandwidth to its center frequency (percentage). When the bandwidth is larger than 100% it is measured as the ratio of the upper frequency to the lower frequency of the band. For example, the 2:1 antenna has one octave bandwidth. An antenna pattern is a graphical representation of the radiation properties of the antenna as a function of space coordinates. Pattern control is the capability of intentionally modifying the pattern, for instance via antenna geometry manipulation. Reactance is defined as a measure of the opposition of capacitance and inductance to current.
There are two types of reactance: capacitive reactance and inductive reactance. Capacitive reactance is a function inversely proportional to the frequency and the capacitance. Inductive reactance is a function proportional to the frequency and the inductance. Total reactance is a function given by the difference between the inductive reactance and the capacitive reactance.
Antennas in the prior art include dipole antennas, helical antennas, loop antennas, and parabolic antennas. FIG. 1A shows a dipole antenna in the prior art. The dipole antenna can be used in communications. The dipole antenna 100 includes two conductors (e.g., a first pole 110, second pole 120) and an antenna feed 150 (e.g., a center feed element). Electric current flow in the dipole antenna from the feed generates of a first current flow 115 in the first pole 110 of dipole antenna 100, and second current flow 125 in the second pole 120 of dipole antenna 100. A common dipole is the half-wave dipole (e.g., a wire of total length equal to half the wavelength). The theoretical maximum gain of a half-wave dipole is 2.15 dB. The fundamental 10-12% bandwidth of a straight half-wavelength cylindrical dipole antenna is a weak function of the ratio of the length of the dipole to its diameter and the reactance as function of frequency. To fit in the small space available, and to comply with stealth requirements, the dipole antenna has to be reduced in size and become electrically small antennas that do not work at resonance (non-resonant dipole). Dipoles that are designed to be much smaller than the wavelength of operation, have a very low radiation resistance and high capacitive reactance that makes them inefficient. The (matched) bandwidth of small dipoles drastically decreases from 10-12% to 0.1% and less.
FIG. 1B shows a helical antenna 130 according to the prior art. A helical antenna 130 has a conducting wire 131 wound in the form of a helix. Helical antennas 130 can operate in one or two principal modes: normal mode (broadside) or axial mode (end-fire). In the normal mode, the dimensions of the helix are small compared with the wavelength. The far field radiation pattern is similar to an electrically short dipole or monopole. Normal mode helical antennas 130 tend to be inefficient radiators and are typically used for mobile communications where reduced size is a critical factor. Helical antennas 130 possess erratic impedance behavior at low frequencies, especially for short helixes with many turns operating in the normal mode. In the axial mode, the helix dimensions are at or above the wavelength of operation. The helical antenna 130 produces circular polarization. Antenna size makes helical antennas 130 unwieldy for low frequency operation, so they are commonly employed only at frequencies ranging from VHF (e.g., about 30 MHz to about 300 MHz) up to microwaves. The axial mode helical antenna has a very directive antenna beam, not appropriate for communications, where a wide beam is required. The helical antenna has about 3-15% bandwidth.
FIG. 1C shows a loop antenna 140, according to the prior art. The loop antenna 140 is an alternative solution to the optimization of the volume occupied for RF communications. Even though the loop antenna 140 is overall smaller than a whip antenna resonating at the same frequency (e.g., the diameter of the loop is about λ/10), it is not practical since it can require assembly, has a very narrow bandwidth, and works well only when very close to the ground. The resonant loop has 10-15% bandwidth. The magnetic loop, however, poses serious health risks for the human body when exposed to its concentrated radiated field.
FIG. 1D shows a parabolic antenna 145, according to the prior art. The parabolic dish antenna 145 has a gain that is mainly a function of its diameter 146 and operating frequency. The parabolic antenna has a narrow beam (antenna pattern) and is desirable for applications that require directive antennas and high gain. For instance, approximate gain and a 3 dB beam angle of a 3 meter parabolic dish are 22 dB at 500 MHz with a 3 dB beam angle of 14 degrees, and 28 dB at 1 GHz with a 3 dB beam angle of 7 degrees. The bandwidth of the parabolic dish antenna 145 is equal to the bandwidth of the feeding element (e.g., the horn). The parabolic dish antenna 145 has good gain and wide bandwidth, but is bulky and needs precise mechanical steering for proper pointing.